JPH0738182A - Light amplifier - Google Patents

Abstract

PURPOSE:To prevent the fluctuation of exciting beam power to be inputted to a light amplifier by inserting a light isolator between the exciting beam source and an optical coupler. CONSTITUTION:Optical isolators 13 and 14 are inserted between exciting beam sources 1, 2 and a 3dB coupler 4 so as not to cause injection synchronism. As another method, optical fiber which is much longer than the coherence length of the exciting beam sources 1 and 2 is inserted between the exciting beam sources 1, 2 and the 3dB coupler 4 so as to allow the coherence of the two beams inputted to the 3dB coupler 4 to be sufficiently low even when the injection synchronism has occurred. As the other method, FM modulation can be applied to the exciting beam sources 1 and 2, and the emission spectrum width can be widened to the same width of the injection synchronism or wider so as to prevent injection synchronism. Thus, the gain fluctuation which has occurred by the conventional constitution when rare earth added optical fiber is used is suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pumping method for an optical amplifier in which a rare earth element is added to an optical fiber to have an amplifying action, and more particularly, a redundant pumping method for pumping a plurality of optical fiber amplifiers by a plurality of pumping light sources. The present invention relates to stabilization of pumping light power in a generalized configuration.

[0002]

2. Description of the Related Art An optical fiber amplifier is a conventional 3R (Resha
Compared to optical repeaters with ping, retiming, and regenerating functions, submarine optical repeater systems have the features that do not depend on transmission speed, that simplification of repeaters is possible, and that large capacity can be achieved by wavelength multiplexing. It is expected to have a wide range of applications from to optical CATV distribution systems for subscribers. An optical fiber amplifier generally has a bandwidth of THz order, and in order to take advantage of its wide band property, it usually amplifies large-capacity information of Gbit / s or more as an optical signal. Very high reliability is required for communication equipment that carries such large-capacity information, and it is an important issue to improve the reliability of the pumping light source, which is the only optically active element used in optical fiber amplifiers. is there. However, a pumping light source is usually required to have a high output of several tens of mW or more, and there is a limit to improving the reliability of a single unit. Therefore, a redundant configuration using a plurality of pumping light sources is usually adopted.

As a conventional redundant configuration of this type, there is, for example, one shown in US Pat. No. 5,173,957, and FIG. 9 is a block diagram of the conventional redundant configuration shown in the above-mentioned document. In FIG. 9, 1 and 2 are pumping light sources, 3 is an output stabilizing drive circuit for pumping light sources, 4 is a 3 dB coupler, 5 and 6 are optical multiplexers, 7 and 8 are rare earth-doped optical fibers, and 9 and 11 are Signal input terminals 10 and 12 are signal output terminals. The operation will be described. The pumping light sources 1 and 2 are driven by the output stabilizing drive circuit 3 so that, for example, the monitor photodiode current built in the pumping light sources 1 and 2 becomes constant, and temperature, power supply voltage fluctuations, etc. occur. Even so, an almost constant optical output is input to the 3 dB coupler 4. The 3 dB coupler 4 outputs 50% of the power of the input light to each output port according to the desired characteristics when the lights having no correlation with each other are input as the input, and the rare earth addition is performed via the optical multiplexers 5 and 6. Excitation light is supplied to the optical fibers 7 and 8. An inversion distribution is formed in the rare earth-doped optical fibers 7, 8 by the excitation light, and the signal input terminals 9, 1,
The weak optical signal input from 1 is amplified by a predetermined amplification degree and output from the signal output terminals 10 and 11. According to this configuration, for example, even if the excitation light source 1 deteriorates and stops emitting light, the excitation light from the excitation light source 2 causes 50
It is possible to operate at a pumping light power of%, and the reduction in gain can be suppressed to a non-problematic level by the line design of the system. When the rare-earth-doped optical fiber 7 is pumped only by the pumping light source 1 without using the 3 dB coupler 4, the rare-earth-doped optical fiber 7 is stopped when the pumping light source 1 stops emitting light.
Operates as a saturation absorber, has a large loss with respect to an input signal of small signal power, and the line through the rare earth-doped optical fiber 7 is cut off. In this way, the reliability of the system can be improved by using the redundant configuration. The above is the operation when the 3 dB coupler 4 performs an ideal operation as a 50% power distributor. That is, since the pumping light sources 1 and 2 are originally separate light sources, the phases independently vary even when their frequencies are equal, and the averaging is performed in a time sufficiently longer than the coherence time of the pumping light sources 1 and 2. The branching ratio of the 3 dB coupler 4 is constant. The coherence time of a semiconductor laser generally used as a pumping light source is 1 μS or less, and the erbium-doped optical fiber most commonly used as the rare earth-doped optical fibers 7 and 8 responds to fluctuations in pumping light power. It is sufficiently smaller than the minimum value of the constant of 10 μS. Therefore, within the response time constant of the rare earth-doped optical fibers 7 and 8, the pumping light power becomes constant and the gain does not change. However, the 3 dB coupler 4 which is the key component of the redundant configuration shown in FIG.
When two lights having the same frequency are input, the branching ratio may change depending on the phase relationship between the two lights input, and as a result, the gains of the rare earth-doped optical fibers 7 and 8 may change. When the pumping light sources 1 and 2 are coupled by the imperfect linearity of the 3 dB coupler 4 or the reflection from the device connected to the output port of the 3 dB coupler 4 and the injection locking state is reached, the single coherence The phase relationship may be maintained for a long time, which is longer than the time.

FIG. 10 is a diagram showing an emission spectrum of the excitation light source when injection locking occurs. In the figure, 101 is the emission spectrum of the excitation light source 1 when injection locking is not occurring, 102 is the emission spectrum of the excitation light source 1 when injection locking is occurring, and 103 is the injection locking pull-in width of the excitation light source 1. , 104 are the emission spectra of the excitation light source 2. When a weaker optical power than the excitation light source 2 is coupled to the excitation light source 1 and the oscillation frequencies f2 and f1 are sufficiently close to each other, for example, in the literature (edited by the Society of Applied Physics, "Basics of Semiconductor Laser")
58p (62), the excitation light source 1 stops the oscillation of the frequency f1 and outputs the light of the frequency f2. This phenomenon is called injection locking. At this time, the frequencies of the light input and the light output to the excitation light source 1 match (f1 moves to f2 as shown in FIG. 10), and the phase is the frequency difference between the two before injection locking (FIG. 10). Then, the phase difference is maintained from -90 degrees to +90 degrees (coherent state). Therefore, when injection locking occurs, the branching ratio of the 3 dB coupler 4 varies depending on the phase difference of the input light. Injection locking occurs when f1 and f2 satisfy the following conditions. | f1-f2 | <Δf (1) where Δf is the injection locking pull-in width, which is a value proportional to the electric field intensity of the injection light from the excitation light source 2 input to the excitation light source 1, Can be represented.

[0005]

[Equation 1]

Here, Pi and Pl are the excitation light source 1 respectively.
Internal injection light power and self-excited oscillation power, τp is a photon lifetime in the internal resonator of the excitation light source 1, and R1 is a front surface reflectance of the excitation light source 1. In general, a high-power semiconductor laser has a photon lifetime τp of about 3 ps. Also, the front reflectance R
1 is a few percent for high power semiconductor lasers. 3dB coupler 4
Assuming that the linearity is 60 dB and the front face reflectance R1 is 3%, the injection locking pull-in width Δf is 138 MHz. The emission spectrum line width of a high-power semiconductor laser is about several MHz, and it is clear that injection locking is highly likely to occur. FIG. 11 is a diagram showing a 3 dB coupler branch ratio variation characteristic when injection locking occurs,
In the figure, 105 and 106 are branching ratios of two output ports of the 3 dB coupler 4. When the phase difference between the lights input from the pumping light sources 1 and 2 is ψ, the branching ratios LA and LB of the through port A and the cross port B viewed from the pumping light source 1 can be expressed by the following formulas.

[0007]

[Equation 2]

As can be seen from the third equation, when two lights having coherence are input, the branching ratio of the 3 dB coupler 4 becomes a function of the phase difference ψ of the light, and when ψ is 90 degrees, all the optical powers are It is output from port B, and when ψ is −90 degrees, all the optical power is output from port A. The phase difference ψ of light is determined by the excitation light source 1, the 3 dB coupler 4, the fiber length L connecting the excitation light source 2, and the difference f1-f2 between the self-oscillation frequencies of the excitation light sources 1 and 2. The fiber length L is constant, but the oscillation frequencies f1 and f of the excitation light sources 1 and 2 are
2 fluctuates randomly, and is the main factor of the fluctuation of the optical phase difference ψ. Due to the random fluctuations of the oscillation frequencies f1 and f2, the branching ratios 105 and 106 of the output ports A and B show complementary random fluctuation characteristics. Self-oscillation frequency f
The random fluctuation of 1 and f2 includes a very low frequency component, and the rare earth-doped optical fiber 7,
A gain of 8 will respond and will vary. This gain variation is characteristic in that the gains of the rare earth-doped optical fibers 7 and 8 are complementary.

[0009]

As described above, in the conventional redundant configuration of the pumping light source, injection locking occurs due to the coupling of the pumping light sources, and two coherent light beams are generated.
Since the pumping light power input to the 3 dB coupler and input to the rare-earth-doped optical fiber fluctuates, there is a problem that the gain itself of the rare-earth-doped optical fiber also fluctuates as a result.

[0010]

The present invention has been made to solve the above problems, and an optical isolator is inserted between a pumping light source and a 3 dB coupler so that injection locking does not occur. is there. As another method, even if injection locking occurs, the excitation light source and 3 dB are set so that the coherence of the two lights input to the 3 dB coupler is sufficiently low.
An optical fiber that is sufficiently longer than the coherence length of the pumping light source is inserted between the couplers. As another method, FM emission is applied to the excitation light source so that the injection locking does not occur, so that the emission spectrum line width is equal to or wider than the injection locking width. As another method, the polarization state of the output light of the pumping light source is changed to right circular polarization and left circular polarization so that interference does not occur in the 3 dB coupler even when injection locking occurs.

[0011]

According to the present invention, it is possible to suppress the gain fluctuation of the rare earth-doped optical fiber generated in the conventional structure, and to realize stable operation while taking advantage of the high reliability due to the redundant structure of the pumping light source. Is.

[0012]

Embodiment 1 FIG. 1 shows a first embodiment of the invention, in which 13 and 14 are optical isolators. The same parts as those of the conventional example are designated by the same reference numerals and the description thereof will be omitted. The operation will be described. The optical isolators 13 and 14 usually have a forward insertion loss of 1 dB or less and a reverse insertion loss of about 40 dB, and keep the loss of pumping light power output from the pumping light sources 1 and 2 small while pumping light. It has the effect of weakening the coupling between the light sources 1 and 2. As a result, the injection locking pull-in width Δf can be reduced. According to the second equation, the optical isolators 13 and 14 are
If the reverse insertion loss is 40 dB and the coupling of the pumping light sources 1 and 2 due to the straightness of the 3 dB coupler 4 is 60 dB, the injection locking pull-in width Δf is 1.38 MHz, which is the emission spectrum line of a normal high-power semiconductor laser. The value is less than the width, and the probability of injection locking is extremely small. Figure 2 shows the measured values of the 3dB coupler branch ratio variation versus return loss characteristics. Here, the return loss is the optical power P2 injected from the excitation light source 2 to the excitation light source 1.
And the output light power P1 of the pumping light source 1, ie, 10 log
It is defined by P1 / P2, and the larger the return loss, the smaller the coupling between the excitation light sources 1 and 2. 3 dB
The coupler branch ratio variation amount was defined by the peak value of the branch ratio variation in FIG. As can be seen from FIG. 2, the greater the return loss, the smaller the change in the branching ratio, and the return loss is
The fluctuation of the branch ratio disappears at 76 dB or more. This is to experimentally verify the relationship between the injection locking pull-in width and the coupling between the pumping light sources, and shows that the branching ratio variation can be sufficiently suppressed by inserting the optical isolator. In the above description, the case where the pumping light source 1 is injection-locked with the optical input of the pumping light source 2 has been described. However, conversely, the pumping light source 2 may be injection-locked with the pumping light source 1. Considering such cases, as shown in Fig. 1, pumping light sources 1, 2 and 3 dB
An optical isolator is inserted between both the couplers 4.

Example 2. FIG. 3 is a diagram showing another embodiment of the invention. In the figure, reference numeral 15 is an optical fiber, the length of which is set longer than the coherence length of the excitation light sources 1 and 2. The operation will be described. Excitation light source 2 to excitation light source 1
The pumping light source 1 enters the pumping light source 1 into the injection locking state by the injection light, and outputs coherent light to the pumping light. However, the optical path length difference between the two input lights at the input of the 3 dB coupler 4 becomes large due to the insertion of the optical fiber 15,
The phase relationship between the two input lights in the 3 dB coupler 4 becomes random (becomes incoherent), and the interference becomes small enough to be ignored. That is, the light output from the excitation light source 2 is input to the excitation light source 1 via the first path that is directly input to the 3 dB coupler 4 and the 3 dB coupler 4, is reflected and amplified, and is input to the 3 dB coupler 4. There is a second route to The optical path length of the second path is 2nL longer than that of the first path. Here, n is the refractive index of the optical fiber 15 of about 1.48, and L is the length of the optical fiber 15. If the optical path length difference 2nL is sufficiently longer than the coherence length of the pumping light source 2, the interference of two lights input to the 3dB coupler 4 through the first path and the second path becomes small, and the 3dB coupler 4 The fluctuation of the branching ratio is small.
FIG. 4 is a diagram showing the fiber length dependency of the 3 dB coupler branch ratio variation. The emission spectrum line width of the excitation light sources 1 and 2 used in the measurement is about 6 MHz. Assuming a Gaussian emission spectrum, the coherence length is 32m. It is generally known that the coherence of light decreases exponentially with respect to the difference in optical path length, and if there is an optical path length difference of 5 times the coherence length, coherence almost disappears. As the length L of the fiber 15, 54 m is required. According to FIG. 4, when the length L of the optical fiber 15 is set to about 50 m, the branching ratio variation of the 3 dB coupler 4 can be set to 0.1 dB or less. Further, when the length L of the optical fiber 15 is set to about 80 m, the variation of the branching ratio can be set to 0.04 dB, which is a value that can be ignored, and this configuration is effective.

Example 3. In the above description, the case where the pumping light source 1 is injection-locked with the optical input of the pumping light source 2 has been described. However, conversely, the pumping light source 2 may be injection-locked with the pumping light source 1. Considering such a case as well, if optical fibers 15 and 16 are inserted between the pumping light sources 1 and 2 and the 3 dB coupler 4 as shown in FIG. 5, the branch ratio fluctuation suppression effect is obtained in any case. Have.

Example 4. FIG. 6 is a diagram showing another embodiment of the present invention, in which 17 and 18 are oscillators. The operation will be described. The excitation light sources 1 and 2 use the oscillator 1 for the drive current.
The alternating currents supplied from 7 and 18 are superimposed. Considering the case of a semiconductor laser, this AC current causes
FM modulation is applied with a modulation efficiency of approximately Hz / mA, and the optical spectrum linewidth widens accordingly. For example, assuming that the FM modulation efficiency of the excitation light sources 1 and 2 is 500 MHz / mA, the optical spectrum linewidth of the excitation light sources 1 and 2 is 1.5 G with a modulation current of 3 mAp-p.
It becomes 1 Hz of injection locking pull-in width 138MHz calculated in the conventional example.
The value is 0 times or more. As a result, even if injection locking occurs, the fluctuation of the branch ratio of the 3 dB coupler 4 due to the injection locking can be suppressed to 1/10 or less.

Example 5. Independent oscillator 17, 1
Although the case where 8 is used has been described, the same effect can be obtained by using one oscillator 17 and one phase shifter 19, as shown in FIG. That is, the alternating current output from the oscillator 17 is distributed to the current directly input to the excitation light source 1 and the current input to the excitation light source 2 with its phase delayed via the phase shifter 19. By inserting the phase shifter 19, the emission center frequencies of the excitation light sources 1 and 2 do not change in the same phase, and as a result, the same effect as using two independent oscillators 17 and 18 is obtained. Can be obtained.

Embodiment 6. In the above description, the oscillators 17, 1
Although the oscillation frequency of 8 was not limited, by setting this oscillation frequency to a value higher than the response frequency of the rare earth-doped optical fibers 7 and 8 to the excitation light power modulation,
A higher effect can be obtained. That is, oscillator 1
If the oscillation frequencies of 7 and 18 are higher than the response frequencies of the rare earth-doped optical fibers 7 and 8 for pumping light power modulation, the rare earth-doped optical fibers 7 and 8 will change even if the outputs of the pumping light sources 1 and 2 slightly change due to the modulation current. The gain of is almost unchanged. The generation period of injection locking is also equal to or higher than the response frequency of the rare earth-doped optical fibers 7 and 8, and the pumping light power looks constant to the rare earth-doped optical fibers 7 and 8 and the gain thereof hardly changes.

Example 7. FIG. 8 is a diagram showing another embodiment of the present invention, in which 20 and 21 are quarter-wave plates. The operation will be described. The pumping light sources 1 and 2 input linearly polarized light into the quarter-wave plates 20 and 21, and the incident angles of the output lights of the quarter-wave plates 20 and 21 are opposite to each other. It is set to have circular polarization. An optical fiber having an ordinary circular core has a characteristic of preserving circularly polarized waves. Therefore, the input lights of the 3 dB coupler 4 hold circularly polarized waves that rotate in opposite directions to each other, and even if they have coherence due to injection locking, they do not interfere due to the orthogonality of polarization. As a result, the branching ratio of the 3dB coupler 4 does not change,
The gains of the rare earth-doped optical fibers 7 and 8 are stable.

[0019]

According to the present invention, it is possible to stabilize the pumping light power while maintaining the high reliability by adopting the redundant structure of the pumping light source of the optical amplifier, and to secure the stable operation of the optical amplifier. Have.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment.

FIG. 2 is a characteristic diagram of a 3 dB coupler branch ratio variation vs. return loss.

FIG. 3 is a block diagram showing a second embodiment.

FIG. 4 is a diagram showing fiber length dependency of 3 dB coupler branch ratio variation.

FIG. 5 is a block diagram showing a third embodiment.

FIG. 6 is a block diagram showing a fourth embodiment.

FIG. 7 is a block diagram showing a fifth embodiment.

FIG. 8 is a block diagram showing a sixth embodiment.

FIG. 9 is a block diagram showing a conventional example.

FIG. 10 is a diagram showing an emission spectrum of an excitation light source when injection locking occurs.

[Figure 11] 3dB coupler branch ratio fluctuation characteristic diagram when injection locking occurs

[Explanation of symbols]

 1,2: Pumping light source 3: Output stabilization drive circuit 4: 3dB coupler 5,6: Optical multiplexer 7,8: Rare earth doped optical fiber 13,14: Optical isolator 15,16: Optical fiber 17,18: Oscillator 19: Phase shifter 20, 21: Quarter wave plate

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koji Goto 2-3-2 Nishishinjuku, Shinjuku-ku, Tokyo International Telegraph and Telephone Corporation (72) Haruo Abe 2-3-2 Nishishinjuku, Shinjuku-ku, Tokyo No. International Telegraph and Telephone Corporation (72) Inventor Naoki Norimatsu 2-3-2 Nishishinjuku, Shinjuku-ku, Tokyo Within International Telegraph and Telephone Corporation (72) Inventor Kuniaki Motojima 5-1-1, Ofuna, Kamakura-shi, Kanagawa Mitsubishi Electric Corporation Communication Systems Laboratory (72) Inventor Tadayoshi Kitayama 5-1-1, Ofuna, Kamakura City, Kanagawa Prefecture Mitsubishi Electric Corporation Communication Systems Laboratory (72) Inventor Junichiro Yamashita 5-chome, Ofuna, Kamakura City, Kanagawa Prefecture No. 1 Mitsubishi Electric Corporation Electronic Systems Laboratory (72) Inventor Eiichi Nakagawa 5-1, Ofuna, Kamakura-shi, Kanagawa No. 1 Mitsubishi Electric Corporation Electronic Systems Research Center

Claims (6)

[Claims] 1. A redundant configuration in which an optical fiber doped with a laser active substance such as a rare earth element or a transition metal is pumped by multiplexing and distributing pumping light output from a pumping light source using an optical coupler. In the optical amplifier thus taken, an optical amplifying device, wherein an optical isolator is inserted between the pumping light source and the optical coupler. 2. A redundant structure in which an optical fiber doped with a laser active substance such as a rare earth element or a transition metal is pumped by multiplexing and distributing pumping light output from a pumping light source using an optical coupler. In the optical amplifier thus taken, an optical amplifying device characterized in that an optical fiber longer than the coherence length of the pumping light source is inserted between the pumping light source and the optical coupler. 3. A redundant structure in which an optical fiber doped with a laser active substance such as a rare earth element or a transition metal is excited by multiplexing and distributing pumping light output from a pumping light source using an optical coupler. In the optical amplifier thus taken, an optical amplifying device characterized in that an alternating current is superposed on a drive current of the excitation light source. 4. The optical amplifying device according to claim 3, wherein the phase of the alternating currents superposed on the plurality of pumping light sources is different by using a transmitter and a phase shifter. 5. The frequency of the oscillator is set to a frequency higher than the response frequency for pumping light modulation of an optical fiber doped with a laser active substance such as a rare earth element or a transition metal. The optical amplifying device according to item 4. 6. A redundant configuration in which an optical fiber doped with a laser active substance such as a rare earth element or a transition metal is pumped by multiplexing and distributing pumping light output from a pumping light source using an optical coupler. In the optical amplifier thus taken, an optical amplifying device characterized in that a component for converting a linearly polarized wave into a right circularly polarized wave and a left circularly polarized wave is inserted into the output of the pumping light source.